Many chemical agents are capable of damaging the lungs. However, the potential for pulmonary injury by such agents is not at present predictable and the detection of injury requires the development of lesions capable of identification by hsitopathological procedures. The alveoli and distal airways of the lungs are covered with a very thin film of fluid known as the pulmonary extracellular lining (EL) which may be sampled without excessive danger or discomfort to patient or subject. Since the EL is in intimate contact with the alveolar and bronchial epithelium it is proposed that chemical agents could damage and release epithelial components into the EL. The objectives of this research are the development of biochemical tests for the assessment of the pulmonary toxicity of chemical agents by quantitative measurements of epithelial cell components in the EL of laboratory animals exposed to various classes of pulmonary toxicants.